\documentclass[11pt,letterpaper]{article}

\makeatletter
\newcommand\ifmember[2]{%
	\in@{,#1,}{,#2,}%
	\ifin@
		\expandafter\@firstoftwo
	\else
		\expandafter\@secondoftwo
	\fi
}
\makeatother

\begin{document}

\ifmember{3}{3,4}{True}{False}
\newcommand{\foo}{3,4}
\ifmember{3}{\foo}{True}{False}

\end{document}
